A polysilicon layer doped with impurities has been used as a gate electrode. However, as a semiconductor device has a high degree of integration, the semiconductor device using the polysilicon layer doped with impurities as a gate electrode has a high resistance due to a high specific resistance of the polysilicon layer. Accordingly, the polysilicon layer may not be suitable for a gate electrode. A polycide gate electrode of which a specific resistance is lower than that of a polysilicon layer has been developed. The polycide gate electrode may have a structure in which a refractory metal silicide layer such as a titanium silicide layer or a tungsten silicide layer is formed on a polysilicon layer doped with impurities. However, the polycide gate electrode may not satisfy a resistance level suitable for a gate electrode called for in high integration semiconductor devices.
Recently, a polymetal gate electrode having a low specific resistance is used as a gate electrode. The polymetal gate electrode has a multilayer structure in which a refractory metal layer is formed on a polysilicon layer doped with impurities. The refractory metal layer may include tungsten. However, when the tungsten layer on the polysilicon layer is patterned, the resistance of the tungsten layer is sometimes increased.
FIG. 1 is a cross-sectional view illustrating a conventional method of forming a gate structure. Referring to FIG. 1, a gate insulation layer, a polysilicon layer, a barrier metal layer and a tungsten layer are sequentially formed on a substrate 10. The gate insulation layer, the polysilicon layer, the barrier metal layer and the tungsten layer are patterned by using a silicon nitride layer mask 35 as an etching mask. Accordingly, a gate structure 40 including a gate insulation layer pattern 15, a polysilicon layer pattern 20, a barrier metal layer pattern 25, a tungsten layer pattern 30 and the silicon nitride layer mask 35 are formed on the substrate 10.
When a silicon nitride layer used as the silicon nitride layer mask 35 is formed on the tungsten layer, nitrogen atoms may penetrate into the tungsten layer, so that the resistance of the tungsten layer may be increased. For example, ammonia (NH3) gas can be provided into a process chamber to control the generation of a remaining oxidation source and oxidation of the tungsten layer before forming the silicon nitride layer on the tungsten layer. In this case, nitrogen atoms included in the ammonia gas may penetrate into the tungsten layer to increase a sheet resistance of the tungsten layer. When the sheet resistance of the tungsten layer is increased, a gate structure formed by a following process may have increased resistance. Also, nitrogen atoms included in the silicon nitride mask 35 may diffuse into the tungsten layer to increase the sheet resistance of the tungsten layer.